Support Data URI for meshes/materials

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/155)) by Shane Loretz (Bitbucket: [Shane Loretz](https://bitbucket.org/%7B656e3311-aad9-45a1-aaf7-b0ee0e84b287%7D/), GitHub: [sloretz](https://github.com/sloretz)).**

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Currently meshes and materials can be referred to using a uri like `model://my_model/meshes/mymesh.stl`. If the `` tag supported [data uri](https://en.wikipedia.org/wiki/Data_URI_scheme) The mesh or model could be base64 encoded and made part of the model or world file.

This would enable describing a world with just a single .world file. All the resources would be part of the world file.